U.S. Pat. No. 5,169,591 shows an impact pad for a tundish for continuous casting of steel that is a significant advance over the art. The impact pad as illustrated and described in that patent can substantially eliminate surface turbulence in a continuous casting tundish as well as providing other advantages including minimization of slag entrainment within the liquid steel bath in the tundish, prevention of the breakup of tundish flux cover and reoxidation of the liquid steel bath, and ensuring a proper flow path of the steel within the tundish.
While the particular design of the impact pad actually illustrated in the drawings of the U.S. Pat. No. 5,169,591 patent is ideally suited for certain combinations of tundish design and casting conditions, while for other combinations of tundish design and casting conditions a modification thereof is more desirable. It has been found according to the present invention that particularly for combinations of tundish design and casting conditions where the additional benefits of increased inclusion flotation or reduced volume of mixed steel chemistry upon grade changes are sought, it is desirable to provide an impact pad having an annular (closed) configuration. It has also been found according to the present invention, however, that the annular configuration must be non-uniform. That is the opening provided in the impact pad into which the liquid steel flows must be longer along the long axis of the tundish than it is along the short axis. The preferred configuration of the impact pad is rectangular, although oval or other polygon shapes may be provided as long as they are non-uniform (e.g. not circular or square).
The impact pads according to the present invention uniformly disperse the flow of incoming steel, rather than merely redirecting it. Since the steel flow can be dispersed over a larger area than if a circular or other uniform configuration were used, the result is more uniformly surface directed flow while maintaining the benefits of reduced splash and turbulence. Also due to the non-uniform configuration of the impact pad it presents a larger "target" to the incoming steel than a similar width uniform shaped pad (e.g. circular or square), with the desired results obtained even if the steel stream is imperfectly aligned with the center of the impact pad.
It is also highly desirable that the impact pad according to the present invention have a completely semi-circular, or extended semi-circular, configuration of the flow re-directing portions beneath the top of the pad and between the top of the pad at the opening thereof and the bottom of the pad toward which the steel is directed. This configuration--which utilizes no corners--minimizes cracks during manufacture, and reduces the potential for erosion. However it is not necessary that the inner side wall be semi-circular--it can be channel-shaped, and it can have sharp corners.
The impact pads according to the present invention are formed of a refractory composition which is capable of withstanding continuous contact with molten metal, in particular molten steel such as used in continuous casting operations. Usually a standard medium-to-high alumina monolithic refractory, with an alumina content in the range of about 55-85%, is desirable. Where a basic refractory is preferred because of steel chemistry it is preferred that a magnesia based monolithic refractory be utilized, with MgO in the range of about 58-93%.
According to one aspect of the present invention a tundish impact pad formed of a refractory composition capable of withstanding continuous contact with molten steel is provided comprising a base having an impact surface and an endless outer side wall extending outwardly from the base, and fully enclosing an interior space having an upper opening for receiving a stream of molten metal. The outer wall includes an annular inner surface having at least a first portion extending inwardly and upwardly toward the opening so that when a downwardly directed stream of molten steel from a location (e.g. ladle outlet) disposed above the impact pad strikes the impact surface, the stream is directed outwardly toward the annular inner surface and then redirected upwardly and inwardly toward the incoming metal stream.
The annular inner surface typically includes a second portion extending outwardly and upwardly from the impact surface toward the first portion and at least one of the first and second portions is a concave annular surface. Preferably a semi-circular surface, elongated at the ends of the semi-circle, is provided. The interior space may be rectangular or oval in shape, or of another polygon shape, as long as it is non-uniform (that is having a longer dimension along the long axis of the tundish than along the short axis). A vertically oriented annular surface may extend upwardly from the first portion and define the opening.
According to another aspect of the present invention in a tundish for holding a volume of molten steel and having a floor and side walls enclosing a region of impact, and a drain, according to the invention an improvement is provided in the form of an impact pad. The impact pad is preferably as described above.
According to yet another aspect of the present invention a method of reducing turbulence and high velocity flow of molten metal (steel) in a tundish is provided. The method comprises the following steps: Providing an impact pad within the tundish, the impact pad including an endless outer side wall extending upwardly therefrom and fully enclosing an interior spacing having an upper opening for receiving a stream of the molten metal, the outer wall including an annular inner surface having at least a first portion extending inwardly and upwardly toward the opening. Directing an incoming stream of molten metal vertically downwardly into the tundish and against the impact pad from a location disposed above an upper edge of the impact pad to create a bath of molten metal in the tundish. Reversing the stream into a vertically upward and inward direction toward the incoming stream. And, creating generally radially outward flows of the molten metal in the tundish on all sides of the incoming stream such that the flows are each directed away from the incoming stream toward the surface of the bath of molten metal.
According to yet still another aspect of the present invention a tundish impact pad is provided comprising the following elements: A body of refractory material capable of withstanding continuous contact with molten steel during the entire use cycle of a tundish. The body comprising a base having an impact surface, an endless outer sidewall extending upwardly from the impact surface, a top surface substantially parallel to the impact surface and connected to the sidewall and defining a non-uniform opening therein, the non-uniform opening having a long dimension and a short dimension perpendicular to the long dimension. And, the sidewall having an interior face which is semicircular about an axis substantially parallel to the impact surface around the entire extend thereof, so that molten steel contacting the impact surface flows outwardly, then is turned inwardly and directed upwardly by the sidewall interior face, and then flows out the opening. The opening is typically substantially rectangular or substantially oval and the body of refractory material preferably comprises a monolithic refractory having about 55-85% alumina, or about 58-93% MgO. The outer side wall may be tapered inwardly at opposite ends thereof, and it may include first and second ears extending outwardly from the opposite ends at said base, which ears facilitate mounting of the body in place in a tundish.
According to another aspect of the invention a tundish assembly is provided which comprises the following elements: A tundish including a floor, a plurality of sidewalls, an open top, and a plurality of outlets in the floor spaced from a central portion thereof, the tundish having a long dimension and a short dimension. And, an impact pad disposed on the floor at the central portion thereof, the impact pad comprising: a body of refractory material capable of withstanding continuous contact with molten steel during the entire use cycle of a tundish; the body comprising a base having an impact surface, an endless outer sidewall extending upwardly from the impact surface, a top surface substantially parallel to the impact surface and connected to the sidewall and defining a non-uniform opening therein, the non-uniform opening having a long dimension and a short dimension perpendicular to the long dimension; and the sidewall having an interior face which is semicircular about an axis substantially parallel to the impact surface around the entire extend thereof, or channel-shaped, so that molten steel contacting the impact surface flows outwardly, then is tuned inwardly and directed upwardly by the sidewall interior face, and then flows out the opening. And, wherein the long dimension of the non-uniform opening is substantially aligned with the long dimension of the tundish, and the short dimension of the non-uniform opening is substantially aligned with the short dimension of the tundish.
It is the primary object of the present invention to provide tundish impact pads which are very desirable for many different types of tundishes. This and other aspects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.